Fractal properties of neurophysiologic signals in rat somatosensory cortex

Abstract

While it is generally accepted that alternations in neuronal activity influence changes in CBF 1, 2, the common analytical approaches usually neglect the small time-dependent variations in the neurophysiological signals and treat them as noise. It remains to be verified whether or not the measured fluctuations or variations (i.e., inhomogeneities) in neuronal activity can affect noise-like fluctuations in CBF. The inhomogeneities in the neuronal and vascular signals can emerge from independent events, but the possibility of non-linear correlations cannot be excluded. To identify these possibilities in the neuronal and vascular fluctuations, scaled windowed variance (SWV) method 3 was applied to characterize variations of neurophysiological signals measured from layer 4 of rat somatosensory cortex 4. Artificially ventilated rats (male, Sprague-Dawley, n=15) were anesthetized with a-chloralose (40 mg/kg/hour). Dynamic changes in CBF and electrical activity were measured by laser Doppler flowmetry (LDF) and extracellular Tungsten microelectodes, respectively, during rest and forepaw stimulation (0.3 ms, 2 mA, 3 Hz, 30 s). The extracellular signals were filtered appropriately to be separated into field and action potentials (FP, AP) and were represented by inter-peak intervals for SWV analysis (see figure 1). The perfusion data were analyzed by amplitude variations 4. We hypothesized that fluctuations in these multi-modal signals could be either random or time-scale invariant (i.e., fractal). The fractal nature of the signal, obtained from SWV analysis 4, was characterized by the Hurst coefficient (H). The CBF signals were fractal and anti-persistent (i.e., the step-by-step changes in the signal were anti-correlated with H 0.5) for all conditions (0.710.13, 0.610.1, 0.710.13, respectively). In contrast the FP signals proved to be random (H=0.5) during pre- and post-stimulus conditions (0.510.06, 0.520.07, respectively) and during stimulation the signals became anti-correlated (H=0.410.11). In summary, during sensory stimulation the fractal properties were partly modified for the electrical signals but not in the blood flow response. The CBF and AP signals appeared more consistent in their deterministic behavior and may be important for physiological modeling 1, 2. These results demonstrate that the apparently random fluctuations in local CBF are related to the neuronal signals, and FP and AP are in fact realizations of complex processes of specific correlation structure 5 that are influenced by sensory-induced functional activity of the brain.